The present application finds particular application in conjunction with hybrid commercial vehicles and will be described with particular reference thereto. It is to be appreciated, however, that it may also find application in conjunction with power supplies for other circuits with high capacitance such as stereo amplifiers, motor controllers, and the like.
Many commercial vehicles include a pneumatic system for controlling brakes and other functions. A compressor cycles on and off to maintain pressure in a pneumatic reservoir within preselected pressure limits. Typically, the compressor is driven directly from the internal combustion engine.
One problem in driving the compressor directly from the internal combustion engine is that the internal combustion engine must be sized sufficiently large not only to propel the vehicle fully loaded, but also to power the compressor and other accessories concurrently. A smaller internal combustion engine can be utilized and fuel saved if the compressor and other accessories are powered electrically rather than directly by the internal combustion engine.
To drive the compressor, a relatively powerful electric motor is needed. To keep the physical size of the motor relatively small, and for other reasons, the power bus of a hybrid vehicle is normally several hundred volts. The motor, typically a three-phase brushless DC type, when turned on, draws a significant current, e.g. on the order of 20 amps. To limit the surge current, others have proposed using resistors and a power switch. However, at currents of this magnitude, the power loss (I2R where I is current and R is resistance) is substantial. After the capacitors are charged, a switch can be used to bypass the resistors. However, a solid-state switch capable of carrying 20 amps or greater is again problematic.
The present application discloses a capacitor pre-charging circuit suitable for use in hybrid commercial vehicles and other applications with high surge currents at high voltages.